Sugars from starch, in the form of concentrated dextrose syrups, are being produced at the rate of several million tons per annum as of the date hereof by a two stage enzyme catalyzed process: (1) liquefaction of the solid starch with an alpha-amylase, then (2) saccharification of the resulting starch hydrolysate with a glucoamylase. Much of the dextrose syrup produced commercially is then enzymatically isomerized to a dextrose/fructose mixture known as isosyrup.
The saccharification stage to which this invention is directed has long been recognized to be deficient in certain regards. In particular, the glucoamylases available to the art catalyze both saccharification and dextrose reversion reactions e.g., of dextrose into isomaltose. Existence of reversion reactions has limited the saccharification of starch hydrolysates into dextrose to not more than about 96% by wt. of dextrose on dry solids basis (hereinafter termed DX) in syrups having at least 30% solids by wt. In an effort to further increase the DX value the art has suggested employment of debranching enzymes (along with the glucoamylase) so as to hydrolyze more efficiently the branched chain oligosaccharides (containing 1,6-alpha-glucosidic bonds) present in the starch hydrolysate. For greater detail on the place of debranching enzymes in maltodextrin saccharification, reference is made to Allen et al. "Technology and Use of Debranching Enzymes," Food Technology, May 1975, pp. 70-80.
A process employing pullulanase is described by Hurst in U.S. Pat. No. 3,897,305. Isoamylase, whose use forms the subject matter of this invention is given but passing mention in U.S. Pat. No. 3,897,305. However, Allen et al. and numerous patents such as for example, U.S. Pat. Nos. 3,677,896 and 3,922,196 recommend employment of isoamylase together with a maltogenic beta-amylase in saccharifications when the objective is a high maltose content syrup.
Employment of pullulanase together with glucoamylase during saccharification of a 30% or more concentrated syrup will increase the DX value by 1-2%, but there are certain disadvantages when using pullulanase. The saccharification is usually conducted at a pH where the glucoamylase is only 60-30% active (e.g., about pH 5.5-6.0) whereas saccharification with glucoamylase alone is carried out at significantly lower pH levels, e.g., pH 4.0.
It has now been found that acidophilic isoamylases, such as Pseudomonas amyloderamosa isoamylase, which has a pH optimum of about 3.0-4.5, are almost ideal for combined use with glucoamylase during saccharification. Overall, one major result of the pH match is that the dose of glucoamylase can be reduced considerably, and a syrup with higher DX may be produced than is attainable with glucoamylase alone.